1. Field of the Invention
The present invention relates to a scanning microwave microscope and a microwave resonator used in a two-dimensional image processing system for electrical properties of conductive material, insulating material or thin film material of a semiconductor device or the like on the order of nanometers with a high resolution.
2. Description of the Related Art
A first prior art scanning microwave microscope is constructed by a microwave resonator including a probe having a sharp end, so that the sharp end is in proximity to a sample while the sample is scanned by the sharp end, thus obtaining a two-dimensional image of an amount regarding an impedance of the sample. In this case, a xcex/4 coaxial resonator having a sharp center conductor is used, and an amount relating to a resonant state is an amount relating to the difference in phase between a microwave detected by an antenna within the resonator and an excited microwave (see; C. Gao et al., xe2x80x9cHigh Spatial Resolution Quantitative Microwave Impedance Microscope by a Scanning Tip Microwave Near-field Microscopexe2x80x9d, Appl. Phys. Lett. 71(13), pp.1872-1874, September 1997; and C. Sao et al., xe2x80x9cQuantitative Microwave Near-field Microscopy of Dielectric Propertiesxe2x80x9d, Review of Scientific Instruments, Vol. 69, No. 11, pp. 3846-3851, November 1998).
A second prior art scanning microwave microscope is constructed by a coaxial multi-stage resonator including a probe having a sharp end, so that the sharp end is in proximity to a sample while the sample is scanned by the sharp end, thus obtaining a two-dimensional image of an amount regarding a resonant frequency or a Q-value of the sample. In this case, a reflected power is taken out of the resonator via a directional coupler, and the above-mentioned amount is obtained by detecting the reflected microwave power (see: C. P. Vlahacos et al., xe2x80x9cNear-field Scanning Microwave Microscope with 100 xcexcn Resolutionxe2x80x9d, Appl. Phys. Lett. 69(21), pp. 3272-3274, November 1996; C. P. Vlahacos et al., xe2x80x9cQuantitative Topographic Imaging Using a Near-field Scanning Microwave Microscopexe2x80x9d, Applied phys. Lett. 72(14), pp. 1778-1780, April 1998; and D. E. Steinharier et al., xe2x80x9cImaging of Microwave Permittivity, Tunability, and Damage Recovery in (Ba, Sr) TiO3 Thin Filmsxe2x80x9d, Applied Phys. Lett. 75(20), November 1999).
A third prior art scanning microwave microscope is constructed by a strip-line type xcex/4 resonator having a tapered line or a probe, so that the sharp end thereof is in proximity to a sample while the sample is scanned by the sharp end. A detected amount is a reflected power or a reflection coefficient S1: from the resonator at a microwave frequency close to the resonant frequency (see: M. Tabib-Azar et al., xe2x80x9c0.4 xcexcm Spatial Resolution with 1 GHz (xcex=30 cm) Evanescent Microwave Probexe2x80x9d, Review of Scientific Instruments, Vol. 70, No. 3, pp. 1725-1729, March 1999; M. Tabib-Azar et al., xe2x80x9cNondestructive Superresolution Imaging of Defects and Nonuniformities in Metals, Semiconductors, Dielectrics, Composites, and Plants Using Evanescent Microwavesxe2x80x9d, Review of Scientific Instruments, Vol. 70, No. 6, pp. 2783-2791, June 1999; M. Tabib-Azar et al., xe2x80x9cNovel Hydrogen Sensors Using Evanescent Microwaves Probexe2x80x9d, Review of Scientific Instruments, Vol. 70, No. 9, pp. 3707-3713, September 1999; M. Tabib-Azar et al., xe2x80x9cNovel Physical Sensors Using Evanescent Microwaves Probexe2x80x9d, Review of Scientific Instruments, Vol. 70, No. 8, pp. 3381-3385, August 1999; M. Tabib-Azar et al., xe2x80x9cTransient Thermography Using Evanescent Microwaves Microscopexe2x80x9d, Review of Scientific Instruments, Vol. 70, No. 8, pp. 3387-3390, August 1999; and M. Tabib-Azar et al., xe2x80x9cReal-time Imaging of Semiconductor Space-charge Regions Using High-Spatial Resolution Evanescent Microwaves Microscopexe2x80x9d, Review of Scientific Instruments, Vol. 71, No. 3, pp. 1460-1465, March 2000.
Scanning capacitance microscopes are different from scanning microwave microscopes for convenience; however, they are essentially similar. That is, a scanning capacitance microscope is constructed by a strip-line resonator and a capacitance sensor having an excitation line and a receiver line coupled to the resonator. A conductive probe used in an atomic force microscope is connected to a resonator line. Then, two-dimensional data relating to the power at a frequency close to the resonant frequency is displayed while a sample is scanned. A commercially-available conductive probe along with a cantilever is manufactured by microfabrication method which performs a metal coating process upon monocrystalline silicon.
In the above-described prior art scanning microwave microscopes, since the end of a center conductor of the resonator is sharpened and is used as a probe, the structure of the resonator and its center conductor can be simply and precisely formed, and also, can be simplified for high frequencies. However, since the distance between the sharp end and the sample is not controlled, problems may occur due to the large dependency of signals generated from the resonator upon the distance between the sharp end and the sample when the sharp end is in proximity to the sample. Also, since the end of the center conductor used as an end of the probe, the resolution is limited by the radius of the center conductor. Further, when the sharp end is abraded, the entire resonator has to be replaced with another resonator.
On the other band, in the above-described scanning capacitance microscope, the cantilever is used for detecting the location of the sharp end in the same way as in the atomic force microscope, so that the distance between the sharp end and the sample can be detected at a high precision of about 1 nm. However, since a structure of the cantilever, the sharp end and the probe, a holder for holding the probe is complex, and the probe made of composite material of silicon and metal adapted to high frequencies is complex. Therefore, a complex electric field is generated within the resonator to cause complex reflected microwaves therein, so that the amount relating to the resonant state is not always sensitive to interference between the sharp end and the sample. As a result, in an extremely high frequency region such as a milliwave region, the microwave wavelength is close to a size of the structure, so that it is impossible to determine an observed resonant mode. Additionally, a change of the resonant state depending upon the specification of the holder for mounting the probe on the resonator may make it difficult to use the scanning capacitance microscope.
Thus, in the above-described prior art microscopes, the simplicity of the structure of the resonator including the probe and the control of the distance between the sharp end of the probe and the sample are insufficient. That is, in the prior art scanning microwave microscopes including a microwave resonator having a sharp end and a detector for detecting an amount relating to the resonant state of the, resonator so as to display this amount while the sample is scanned by the sharp ends when the distance between the sharp end and the sample is controlled, it is difficult to steadily maintain this distance at a definite small value.
It is an object of the present invention to provide a scanning microwave microscope capable of controlling an average distance between a sharp end of a microwave resonator and a sample at a definite value, thereby obtaining a high resolution.
Another object is to provide a microwave resonator used in the above-mentioned scanning microwave microscope.
According to the present invention, in a scanning microscope including a microwave resonator, an exciting unit for exciting the microwave resonator, a first detecting unit for detecting a first detection amount relating to a resonant state of the microwave resonator, a sharp end coupled to a center conductor of the microwave resonator, and a display unit for displaying the first detection amount while a sample is scanned by the sharp end, a distance changing unit causes a differential change in a distance between the sharp end and the sample. A second detecting unit detects a second detection amount relating to a change of the first detection amount. A control unit controls the distance between the sharp end and the sample in accordance with the second detection amount, so that an average value of the distance between the sharp end and the sample is brought close to a definite value.
Also, a microwave resonator includes a line section and a replaceable probe provided in the line section. The probe is constructed by a sharp conductive end.